Wireless fidelity (Wireless Fidelity, WiFi) is a technique which enables a personal computer, a hand-held device (such as a PDA or a cell phone), and other terminals to be interconnected in a wireless manner, so as to improve interoperability among wireless network products based on IEEE 802.11 standards.
The most basic network structure of the WiFi includes an access point (Access Point, AP) and several stations (Station, STA) supporting IEEE 802.11 protocols, and the AP provides the STAs with an access service. A wireless signal is used as an interaction medium between the AP and the STAs. The AP periodically broadcasts a beacon (Beacon) message to surrounding areas, and the message includes a lot of information, such as the name of a network which the AP belongs to, a MAC address of the AP, and a period of the Beacon. A STA may obtain information about the AP and the network of the AP through the Beacon, or may obtain a probe response reply from the AP by broadcasting a probe request message to the AP, so as to obtain information about the AP and the network of the AP. When the STA broadcasts the probe request, the STA may specify a dedicated AP (by specifying a dedicated BSSID) to receive the probe request, or may broadcast the probe request to all APs (by specifying a wildcard BSSID). If the STA specifies a dedicated AP to receive the probe request, it is possible that it is the first message initiated by the STA for applying for network access; and if the STA broadcasts the probe request to all APs, it means that the STA only queries for currently available AP resource information.
If the STA wants to communicate with the AP, the STA first finds a network through a Beacon or a probe request/probe response procedure, and then turns into a state in which the STA is associated with the AP through an access procedure. Then the AP and the STA can communicate with each other. The access procedure refers to a series of procedures initiated by the STA to interact with the AP after the STA finds the network. After the interaction procedures are successfully completed, the STA and the AP are in an associated state.
Wireless transmission between the AP and the STAs is performed through a specific frequency, which means that data transmission can be implemented between only one STA and the AP on a wireless medium at the same moment. Therefore, when multiple STAs send access requests to the AP at the same moment, collisions may occur.
Therefore, in order to prevent the collisions, it is necessary to have a collision avoidance mechanism. The collision avoidance mechanism used in the 802.11 standards is called CSMA/CA (carrier sense multiple access with collision avoidance). Contention principles of the CSMA/CA are as follows: 1. before each STA attempts to send a message, the STA first monitors whether a wireless channel is idle; 2. if the STA confirms that the radio channel is idle, the STA randomly generates backoff time within a determined backoff window range; 3. since the backoff time of each STA is random and independent from each other, the backoff time is probably different; 4. the STA continuously senses the channel, and if the channel is idle, after a backoff time unit, the STA decreases a value of the backoff time progressively; and if the channel is busy, the STA pauses decreasing the backoff time, and after the STA confirms that the channel is idle the next time, the STA proceeds to decrease the backoff time value; 5. after the backoff time of the STA is decreased to 0, the STA sends a message; 6. if the STA can receive corresponding confirmation information after sending the massage, it means that at this time, only the STA has the backoff time decreased to 0, obtains a resource in the contention, and succeeds in sending; otherwise, it means that more than one STA has the backoff time decreased to 0 at the same time, so that collisions are caused, and in this situation, the STAs have to contend again.
The random backoff time is generated within a certain range, and the range is called a backoff window. Current technical standards have distinct limits on the size of the backoff window. An upper limit value of the smallest backoff window varies with different physical layer specifications and different requirements on service flow QoS, with 3 (voice data) being the smallest and 63 being the greatest. An upper limit of the greatest backoff window is 1023. In the existing technical solution, all STAs are equal and may contend at the same moment. The STA (or the STAs) having the smallest backoff window will have the backoff time value decreased to 0 at the earliest time, thereby attempting to send a message. If the STA can receive returned confirmation information, it means that the STA succeeds in sending; otherwise, it means that the collisions occur, so the STA has to contend again.
As in current standards, one AP supports only 2007 STAs, the contention strategy not yet has an eminent problem. However, in 802.11ah, the number of STAs supported by one AP is expanded to more than 6000. When the large number of STAs contend for network access at the same time, according to the above backoff window strategy, the situation increases greatly that two or more smallest backoff windows being the same are generated. In this situation, the STAs have the backoff windows decreased to 0 at the same time, and then attempt to send messages at the same moment, which will definitely cause a lot of collisions, thereby affecting overall STA access efficiency.